IOS: is there any way to determine when a UIView appeared after calling view.hidden = NO

I have a UIView with 8 to 10 large images (each loaded in a UIImageView) as subzones. These images are aligned next to each other so that they constitute a real long horizontal image. This view is placed in a UIScrollView so that the user can scroll along this long horizontal image.

Downloading all of these images (from the Internet) takes quite a while, so I decided to put this download code in a subclass of NSOperation. Inside the main operation method, each ImageView is added to the container view.

In this process, the view is hidden, and instead a small rotating image appears (almost similar to the UIActivityIndicatorView). Here is the code:

CategoryViewController.m

- (id)initWithFrame:(CGRect)frame
{
    self = [super init];
    if(self)
    {
        // do some unrelated stuff
        [...]

        loadIndicator = [[UIImageView alloc] initWithImage:img];
        [loadIndicator setFrame:CGRectMake((frame.size.width - width) / 2, (frame.size.height - height) / 2, width, height)];
        [loadIndicator setHidden:YES];

        // this is the view which will contain those large images
        backgroundView = [[UIView alloc] init];
        backgroundView.hidden = YES;

        scrollView = [[UIScrollView alloc] initWithFrame:frame];
        [scrollView setDelegate:(id<UIScrollViewDelegate>)self];
        [scrollView addSubview:backgroundView];  

        [self.view setFrame:frame];
    }
    return self;
}

- (void)viewDidLoad
{
    // do some unrelated stuff
    [...]

    [self.view addSubview:loadIndicator];
    [self.view addSubview:scrollView];

    // setup a custom activity indicator
    loadIndicator.hidden = NO;
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDelegate:self];
    [UIView setAnimationCurve:UIViewAnimationCurveLinear];
    loadIndicatorRotation = 0;
    rotationTimer = [NSTimer scheduledTimerWithTimeInterval: 0.01 target: self selector:@selector(updateRotation:) userInfo: nil repeats: YES];
    [UIView commitAnimations];

    // setup the operation to load all background images
    operationQueue = [[NSOperationQueue alloc] init];
    LoadBackgroundImagesOperation* operation = [[LoadBackgroundImagesOperation alloc] initWithImages:bgImageNames View:backgroundView Frame:frame];

    // register to recieve the finish notification
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onFinishedLoadingBgImages) name:@"BgImagesFinishedLoading" object:operation];

    // Add the operation to the queue
    [operationQueue addOperation:operation];
    [operation release];
    [super viewDidLoad];
}

Main method LoadBackgroundImagesOperation.m

- (void)main
{
    int left = 0;
    int width, height;

    int i;
    for(i = 0; i < imageNames.count; i++)
    {
        if([self isCancelled])
        {
            [[NSNotificationCenter defaultCenter] postNotificationName:@"BgImagesCancelled" object:self];
            return;
        }

        // code to retrieve the url of current image
        [...]

        NSData* data = [[NSData alloc] initWithContentsOfURL:imgURL];
        UIImage* image = [[UIImage alloc] initWithData:data];
        [data release];

        double scalingFactor = frame.size.height / image.size.height;
        width = image.size.width * scalingFactor;
        height = image.size.height * scalingFactor;

        UIImageView* imgView = [[UIImageView alloc] initWithImage:image];
        [image release];

        [imgView setFrame:CGRectMake(left, 0, width, height)];
        [background addSubview:imgView];
        [imgView release];

        left += width;
        [background setFrame:CGRectMake(0, 0, left, height)];
    }
    [[NSNotificationCenter defaultCenter] postNotificationName:@"BgImagesFinishedLoading" object:self];
}

When the operation is completed, I hide the rotating image and show a view containing large images.

(again in CategoryViewController.m)

- (void) onFinishedLoadingBgImages
{
    [scrollView setContentSize:backgroundView.frame.size];

    backgroundView.hidden = NO;
    loadIndicator.hidden = YES;

    if(operationQueue != nil)
    {
        [operationQueue cancelAllOperations];
        [operationQueue release];
        operationQueue = nil;
    }
    [rotationTimer invalidate];
    rotationTimer = nil;
}

The problem is that the simulator takes about 2 seconds and up to 5 seconds on a real device until the images are really visible after the call backgroundView.hidden = NO

I would just keep showing an activity indicator for this time, but I don’t know how to detect what backgroundviewappeared to hide the indicator again.

backgroundview UIViewController viewDidAppear, . ( , viewDidAppear, , , ).

, backgroundview loadIndicator, , , .

, , , , backgroundview .

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